In non-aqueous electrolyte secondary batteries used as electric power sources for portable electronic equipments, the positive electrode includes a lithium-containing transition metal oxide as an active material, while the negative electrode includes a carbon material capable of absorbing and desorbing lithium.
The positive electrode mixture includes an active material and a binder for binding active material particles together. The positive electrode mixture may further include a conductive agent, a thickening agent for facilitating formation of the electrode, or the like. The positive electrode sheet is produced by applying such a positive electrode mixture to a core member made of metal foil or the like.
As the binder, polyvinylidene difluoride (hereinafter referred to as PVDF), polytetrafluoroethylene (hereinafter referred to as PTFE), or modified acrylic rubber dispersed in water, for example, is used, as disclosed in Japanese Laid-Open Patent Publication No. Hei 11-25956.
The positive electrode mixture, however, is often mixed with an organic solvent and then applied to the core member. Thus, the modified acrylic rubber dispersed in water is not commonly used, since inclusion of water in the positive electrode mixture causes gas evolution inside the battery.
PVDF dissolved in the organic solvent tends to coat particles of the active material and the conductive agent, so that PVDF is not distributed effectively between these particles. In order to bind the particles of the active material sufficiently, a large amount of PVDF, for example, 2 to 5 parts by weight of PVDF, is necessary per 100 parts by weight of the active material.
On the other hand, PTFE maintains its particulate form in the organic solvent, but in the manufacturing process of the positive electrode plate, it is subjected to shearing force in rolling process, thereby to produce fibrils. In this case, PTFE functions as the binder when a large amount of the fibrils become entangled with the particles of the active material and the conductive agent. Therefore, a large amount of PTFE is also required for binding the particles.
However, when a large amount of the binder is contained in the positive electrode mixture, the weight of the active material contained in the unit volume of the mixture, that is, active material density, is decreased, so that the capacity of the electrode is decreased. The increase of the binder also results in reduction in porosity of the electrode, thereby lowering the output of the battery.
The use of a large amount of the binder further presents a problem that a large amount of the conductive agent is required in order to ensure that the positive electrode has sufficient electronic conductivity. For example, in case of using polyacrylonitrile dissolved in formaldehyde as the binder, 4 or more parts by weight of the conductive agent becomes necessary per 100 parts by weight of the active material, as disclosed in Japanese Patent No. 3046055. In order to increase the active material density of the positive electrode, however, it is desired to reduce the amount of the conductive agent as well.
On the other hand, when the amount of the binder is reduced in the positive electrode mixture to raise the active material density, it becomes difficult to process the positive electrode mixture. For example, the positive electrode mixture comes off from the core member during the manufacturing process. Therefore, it is difficult to fabricate a wound electrode assembly by using a positive electrode sheet comprising a positive electrode mixture having a high active material density of 3.0 g/ml and over. Further, when the active material density is raised, the strength of the positive electrode is deteriorated, which makes life characteristics of the battery insufficient.